In the field of molecular nano-electronics, semiconductor nano-crystals, nanowires (NWs) and carbon nanotubes (CNTs) are becoming more and more important as components for various electronic devices. These NWs and CNTs are unique for their size, shape and physical properties and have, depending on their electrical characteristics, been used in electronic devices such as e.g. diodes and transistors. Although a lot of progress has been made on both fabrication and understanding of the limits of performance of these NWs and CNTs, there are still key issues to be addressed for potential technological applications. For example, there are still important limitations in processes for doping the NWs and CNTs, which would be required when incorporating such NWs or CNTs in various electronic devices.
Synthesis and doping of p- and n-type germanium (Ge) NWs using gas-phase dopants such as phosphine and diborane has been demonstrated by Greytak et al. in Appl. Phys. Lett. 84 (21), 2004, p. 4176. Intrinsic growth of Ge NWs from gold nanoclusters is initiated at 320° C. and 500 Torr using 1.5% germane (GeH4) in an atmosphere of hydrogen (H2). Elongation of the NW structures is carried out at a reduced temperature of 285° C. After growth, the NW surfaces are doped at 380° C. with either phosphine or diborane in the absence of germane. The doping conditions are chosen to produce a self-limited layer of activated dopant atoms as estimated from atomic-layer-doping studies on planar SiGe. However, with the method described above it is difficult to control the dopant concentration and it may be difficult to obtain low dopant concentrations of lower than 1017 atoms/cm3 in the NWs.
In US 2006/0234519 A1 plasma ion implantation immersion (PIII) is used to dope nanowires and other nano element-based devices on substrates. For example, a method for doping portions of at least one nanowire on a specimen is disclosed which generally comprises enclosing the specimen in a chamber, wherein the specimen includes at least one nanowire thereon having at least one exposed portion, coupling an electrical potential to the specimen; and sourcing a plasma into the chamber, the plasma including ions of a doping material whereby the ions from the plasma implant the at least one exposed portion of the at least one nanowire. However, doping of the nanowires is not performed during growth and thus is difficult to control. This is because it is difficult to implant the whole length of the nanowires due to geometry limitations (small diameter in the order of a few nm and high aspect ratio).
Dilts et al. (Materials Research Society Symposium Proceedings Vol. 832, 2005, pp. 287-292) fabricated high density boron-doped silicon NW arrays within pores of anodized alumina membranes using vapor-liquid-solid (VLS) growth. Boron-doped silicon NWs were synthesized within the pores by VLS growth using silane (SiH4) and trimethylboron (TMB) gas sources. Cui et al. (The Journal of Physical Chemistry, Volume 104, Number 22, Jun. 8, 2000) described the use of laser catalytic growth to controllably introduce either boron or phosphorus dopants during the vapor phase growth of silicon NWs.
However, adding dopant gases such as trimethylboron (TMB) gas to the reaction chamber can cause deformation of the catalyst particle used to catalyze NW growth. Deformation of the catalyst particle may have an effect on the diameter of the nanowire and may, for example, lead to a higher diameter than was required. Furthermore, it is difficult to control the dopant concentration and to obtain low dopant concentrations of lower than 1017 in the nanowire.